1. Field of the Invention
The invention relates to a method for suppressing growth of various types of algae which thrive in bodies of salt water, such as salt water solar ponds.
2. Description of the Prior Art
Artificial salt water solar ponds are presently used as solar collectors in order to provide a source of low grade heat for conversion into electricity. Such ponds have a 3-layer regime: an upper convective wind-mixed layer at the surface with an average salinity of 3-5% and with a depth of 30-50 cm., depending upon wind conditions; an intermediate, nonconvective layer termed a halocline, about 1-1.5 m. deep, with a salinity that increases uniformly with depth from about 5% at the top to about 30% at the bottom; and a lower heat-storage layer, from 3-5 m. deep, depending on the amount of heat storage desired, with a uniform salinity of about 30%.
Solar radiation incident on the surface is absorbed within the layers. Heat absorbed within a stratum of the wind-mixed layer reduces the density of the stratum, and creates buoyant water which quickly reaches the surface, dissipating the absorbed heat into the atmosphere. Thus, the temperature of the wind-mixed layer approximates ambient temperature. However, heat absorbed in the halocline and in the heat-storage layer is trapped in these layers. The halocline is nonconvective because the density of an underlying stratum is so large, compared with the stratum immediately above, that any decrease in density due to a temperature rise in the lower stratum as a consequence of the absorption of solar radiation is insufficient to materially change its buoyancy. As a result, solar radiation establishes a temperature profile that matches the salinity profile in the pond.
In the heat storage layer of the pond, the temperature, while not exceeding the boiling temperature of water, can nevertheless attain sufficient levels such that the water from the heat storage layer can be withdrawn, be used to perform useful work, and then be returned to the heat storage layer.
The clarity of the water in a solar pond, and particularly the clarity of the wind-mixed layer, is of primary importance, because heat absorbed in the wind mixed layer due to its turbidity is entirely lost to the system. A clear wind-mixed layer will permit solar radiation to penetrate into the halocline and even into the heat storage layer, which stores the absorbed heat for useful purposes.
Unfortunately, many microorganisms are capable of thriving even in the concentrated salt water of a solar pond. These microorganisms can thrive near the surface in the wind-mixed layer, or in the halocline of the pond. They can also thrive, and attach themselves to, nets floating on the surface of a pond which serve as wind-breaks and which suppress deepening of the wind-mixed layer. Attachment of algae to the nets decreases the buoyancy of the nets and can cause them to sink, thereby reducing the effectiveness of the nets. The presence of algae on the nets also decreases light transmission through the nets such that the efficiency of the solar pond is reduced.
As mentioned above, algae growing in the wind-mixed layer of a solar pond cause turbidity which also decreases penetration of solar radiation into the pond. Moreover, these surface algae may sink at the end of their growth cycle, thus enriching the sediment at the bottom of the pond with organic matter. This enhances both bacterial activity and the production of metabolic gases in the bottom of the pond which is detrimental to the stability of the pond.
Algae growing along the shoreline of the pond, such as benthic algae, form a dark belt around the sloping sides of the pond where the precipitation of aragonite occurs. If this slimy algal biomass rises from the bottom and floats to the surface of the pond, as it sometimes does, the absorption of solar radiation is adversely affected.
The end result of excessive algal growth is always an increase in the turbidity of the water and a reduction in the depth of penetration of solar radiation into the lower levels of the solar pond, which reduces the effectiveness of the pond. When the algal growth reaches a threshold level, "blooms" are formed at or near the surface of the pond. These are cells scattered in the water which cast shadows in the pond sharply reducing the absorption of the solar radiation.
The conventional approach to reducing excessive algal growth in bodies of salt water is to utilize algacides. Other techniques include the addition of other microorganisms, such as brine shrimp, to devour the unwanted algae, and the elimination of nutrients from the pond and from make-up water, such that algae in the pond die from lack of proper nutrients. These methods often involve complicated procedures and can be very expensive to implement.
A simple and inexpensive method to decrease excessive concentrations of undesirable microorganisms in bodies of water such as solar ponds is thus desirable; and it is an object of the invention to provide a new and improved process for reducing the concentrations of various algae found in a given body of salt water.